Ultra high molecular weight (“UHMW”) polymers have a variety of important commercial uses. For example, UHMW high density polyethylene (“UHMWPE”) may be useful in products including ballistic protection fabrics, joint replacement materials and microporous films (WO 2004/026921A1 and U.S. Pat. No. 4,734,196). When used as battery separators, it is desired that the microporous films have well-balanced shutdown properties, meltdown temperatures, permeability, and pin puncture strength. Some forms of UHMW polypropylene (“UHMWPP”) have been found to be useful in gel spun high melting and high strength fibers, as additives for manufacture of microporous films, and as additives to control melt rheology of lower molecular weight polypropylene.
UHMW polypropylene can be manufactured using a Ziegler-Natta catalyst and dicyclopentyldimethoxysilane as an electron donor. See, e.g., JP 06234811A, JP 06234812A and JP 07292021A. The first of these references discloses a product having an intrinsic viscosity of from 4 to 10 dl/g, and the second discloses a product having an intrinsic viscosity of from 5 to 10 dl/g and a Melt Flow Rate (“MFR”) of less than 0.1 g/10 minutes. Both products are said to be useful for yarns with good break strength and modulus. The product from the third above reference is taught to have an average molecular weight of 1×106.
JP 62022808A discloses producing UHMW polypropylene having a molecular weight of from 2×106 to 5×106 using a Ziegler-Natta catalyst without the dicyclopentyldimethoxysilane electron donor. Similarly, JP 03007704A discloses producing polypropylene and copolymers having a molecular weight greater than 1×106 using a Ziegler-Natta catalyst without the dicyclopentyldimethoxysilane electron donor. JP 02305809A also discloses producing UHMW polypropylene without the electron donor in a Ziegler-Natta catalyzed polymerization.
U.S. Pat. No. 4,413,110 discloses polypropylene having a molecular weight of 2.1×106, which is disclosed as useful for high strength fibers. U.S. Pat. No. 5,070,051 discloses UHMW polyethylene and polyhexene, but not polypropylene, made using Ziegler-Natta catalyst without an dicyclopentyldimethoxysilane electron donor.
EP 0654476A and EP 0790076A disclose polypropylene made using metallocene catalyst. In the first of these reference, the product has a molecular weight of 786,500, a melting point of 159° C. and a molecular weight distribution (“Mw/Mn”) of 2.4. The product of the second reference has a molecular weight of 1.1×106, a molecular weight distribution of 2.5 and a melting point of 159° C.
While polypropylene has been produced using metallocene catalysts, and Ziegler-Natta catalyst both with and without an dicyclopentyldimethoxysilane electron donor, the resulting polypropylene can be difficult to use for producing microporous films, particularly for producing microporous films containing polypropylene and polyethylene. For example, U.S. Pat. No. 6,096,213 discloses a microporous film produced from blends of melt-processable polymers, including blends of polyethylene and polypropylene. The patent discloses that polypropylene is considered melt processable only when it has a melt flow index above 0.2 dg/min.
There is therefore a need for an UHMWPP that can be used to produce microporous films such as those that contain polyethylene and polypropylene, particularly where the polypropylene has a melt flow index of 0.2 dg/min or less.